It is known that gas turbines are machines consisting of a compressor and a turbine with one or more phases, which are connected to each other by at least one combustion chamber.
Air from the outside environment is fed to the compressor to bring it under pressure.
The pressurized air passes through a premixing chamber, terminating with a nozzle or convergent portion. In said chamber, at least one injector feeds fuel which is mixed with the air to form an air-fuel mixture to be burnt.
The fuel necessary for producing combustion is therefore introduced into the combustion chamber by means of a pressurized network. The purpose of said combustion is to cause an increase in the temperature and enthalpy of the gas.
In order to improve the flame stability characteristics, a parallel fuel feeding system, suitable for generating a pilot flame, is also generally envisaged.
U.S. Pat. No. 5,381,652 indicates a combustion system for gaseous fuels in which a main flame is stabilized by means of a pilot device.
In this case, the pilot device injects pilot fuel gas into the combustion chamber, with which the combustion system is equipped.
A duct is present in the combustion system, for premixed air which flows to a first central duct in which there is a first series of radial holes for the fuel gas which feeds the main flame.
In the main duct, the air is premixed with the gaseous fuel fed by injection through a second series of holes, which are situated on cylinders positioned on a section of the main air duct, at the same distance along the perimeter of the section itself.
In this case, the injection of the pilot fuel is effected by means of a first series of radial holes situated at the outlet of the premixing duct, surrounding the main flame.
As this pilot fuel has not been previously mixed with comburent air, it burns in flames mainly controlled by the diffusion process of the fuel in the comburent air, which are consequently called diffusion flames.
Diffusion flames are characterized by the presence of areas in which the temperature is extremely high, therefore causing high polluting emissions.
The emission of nitrogen oxides is, in fact, proportional to the maximum flame temperature and flames of the diffusion type having high temperatures are consequently most responsible for polluting emissions, in particular emission of nitrogen oxides.
One of the disadvantages of the present system is that it has moderate polluting emissions due to the presence of diffusion-type flames.
U.S. Pat. No. 5,666,044 describes the use of pilot devices for stabilizing the main flame, which inject fuel gas through nozzles into the combustion chamber.
Air, necessary for the combustion of the fuel gas injected by means of the pilot device is contemporaneously injected into the combustion chamber, in an area adjacent to each pilot device. Also in this case, the mixing takes place in the combustion chamber, and the flames are consequently of the diffusive type, even if the presence of the high quantity of air in the area adjacent to the injection, reduces the extension of high temperature areas.
A combustion system is also known from the patent application EP 1321715A2, which can be used with liquid or gaseous fuel, in which there are devices which inject the pilot fuel directly into the combustion chamber.
One of the disadvantages of this solution is that it generates significant emissions of polluting agents, also in this case due to the presence of flames of the diffusion type used for stabilizing the main premixed flame.